View source for Earth:Basin Groups

{{Short description|9 subdivisions of the lunar Pre-Nectarian geologic period}}
'''Basin Groups''' refers to 9 subdivisions of the lunar [[Earth:Pre-Nectarian|Pre-Nectarian]] [[Astronomy:Lunar geologic timescale|geologic period]]. It is the second era of the [[Earth:Hadean|Hadean]].

==Definition==
The motivation for creating the Basin Groups subdivisions was to place 30 pre-Nectarian impact basins into 9 relative age groups. The relative age of the first basin in each group is based on crater densities and superposition relationships, whereas the other basins are included based on weaker grounds.<ref name="W87">{{cite book | title =Geologic History of the Moon  | url =http://ser.sese.asu.edu/GHM/ | last = Don Wilhelms | publisher =  U.S. Geological Survey Professional Paper 1348 | year = 1987}}</ref> Basin Group 1 has no official age for its base, and the boundary between Basin Group 9 and the [[Earth:Nectarian|Nectarian]] period is defined by the formation of the Nectaris impact basin.

The age of the Nectaris basin is somewhat contentious, with the most frequently cited numbers placing it at 3.92 Ga, or more infrequently at 3.85 Ga.<ref>{{cite journal | last1 = D. Stöffler and G. Ryder | title = Stratigraphy and isotope ages of lunar geologic units: chronological standards for the inner solar system | year = 2001 | journal = Space Sci. Rev. | pages = 9–54 | volume = 96 | doi = 10.1023/A:1011937020193 | first1 = D. | last2 = Ryder | first2 = G.| bibcode = 2001SSRv...96....9S | s2cid = 117660264 }}</ref> Recently, however, it has been suggested that the Nectaris basin could be, in fact, much older and might have formed at ~4.1 Ga.<ref>{{cite journal |author1=R. Korotev |author2=J. Gillis |author3=L. Haskin |author4=B. Jolliff  |name-list-style=amp | title = On the age of the Nectaris basin | journal=Workshop on Moon Beyond | year = 2002 | pages = abstract 3029}}</ref> Basin Groups are not used as a geologic period on any of the United States Geological Survey lunar geologic maps. Basin Groups 1-9 and the earlier (informal) Cryptic era together make up the totality of the [[Earth:Pre-Nectarian|Pre-Nectarian]] period.


==Relationship to Earth's geologic time scale==
Since little or no [[Earth:Geology|geological]] evidence on [[Astronomy:Earth|Earth]] exists from the time spanned by the Pre-Nectarian period of the Moon, the Pre-Nectarian has been used as a guide by at least one notable scientific work<ref>{{cite book | title = A Geologic time scale 1989 | publisher = Cambridge University Press | year = 1990 |author1=W. Harland |author2=R. Armstrong |author3=A. Cox |author4=L. Craig |author5=A. Smith |author6=D. Smith }}</ref> to subdivide the unofficial terrestrial [[Earth:Hadean|Hadean]] [[Earth:Eon (geology)|eon]]. In particular, it is sometimes found that the Hadean eon is subdivided into the Cryptic era and Basin Groups 1-9 (which collectively make up the [[Earth:Pre-Nectarian|Pre-Nectarian]]), and the [[Earth:Nectarian|Nectarian]] and Lower Imbrian. The first lifeforms (self replicating [[Biology:RNA|RNA]] molecules, see RNA world hypothesis) may have evolved on earth around 4 [[Bya|bya]] during this era.

==See also==
*{{C|Hadean|Hadean eon-related topics}}
* Cryptic era
* Geological time scale (Earth)

==References==
{{reflist|25em}}

==External links==
* [https://web.archive.org/web/20130106085716/http://www.stratigraphy.org/bak/geowhen/index.html GeoWhen Database]

{{Hadean Footer}}
{{Geological history|c}}

[[Category:Hadean|*02]]

{{Sourceattribution|Basin Groups}}